The most minimal example of it I can think of would be a little ridiculous, but it could look like this: interface Edge { } interface Graph { List<Edge

The most minimal example of it I can think of would be a little ridiculous, but it could look like this: interface Edge { } interface Graph { List<Edge> getRoots(); // Returns some ~minimal set of edges with connectivity to all the others List<Edge> getAdjacentEdges(Edge e, boolean tail); } There's no way to directly refer to a vertex here at all (unlike with edges), and yet (since edges have identity) there's enough information to determine the graph structure!